In head up displays such as those used in automotive vehicles for presenting instrument information in the line of sight of the vehicle operator it is necessary to provide an image bright enough for legibility during bright day time conditions. It is known to use a light emissive image source such as a vacuum fluorescent tube. Such tubes produce a blue-green image with limited brightness and resolution. It is desirable, however, to have full color displays with a bright image and high resolution. It is further required for such displays that bright uniform illumination be apparent for off-axis viewing of the display to accommodate movement of the observer's head. For head up displays a viewing angle 15 degrees from the center axis of the LCD without noticeable edge darkening is generally sufficient.
Liquid crystal displays (LCD) afford full color images of the desired resolution but require bright back lighting for projecting an acceptable image onto the windshield or other viewing surface. Intense light sources generally are based on incandescence or plasma discharge and create sufficient heat to result in unacceptably high temperatures of the LCD; according it has been proposed to isolate the lamps from the LCD by fiber optic coupling or lens arrangements. Each of these light coupling approaches introduces light loss elements between the source and LCD requiring even brighter lamps. In addition such elements may impose restrictions on the image uniformity for off-axis projection.